protamine pro·ta·mine (prō'tə-mēn', -mĭn) or pro·ta·min (-mĭn)n. Any of a group of simple proteins found in fish sperm that are strongly basic, are soluble in water, are not coagulated by heat, and yield chiefly arginine upon hydrolysis. In purified form, they are used in a long-acting formulation of insulin and to neutralize the anticoagulant effects of heparin.